Presynaptic α7 and non-α7 nicotinic acetylcholine receptors modulate [3H]d-aspartate release from rat frontal cortex in vitro

The presynaptic nicotinic modulation of glutamatergic transmission in the CNS has been associated with activation of the α7 subtype of nicotinic acetylcholine receptor (nAChR) in sub-cortical regions, whereas in the frontal cortex, non-α7 nAChRs have been implicated. The aim of this investigation was to directly characterise nAChR-evoked release of excitatory amino acids from rat frontal cortex, by monitoring the release of [3H]d-aspartate from superfused synaptosomes or minces. Co-administration of a nAChR agonist with a depolarising stimulus enhanced [3H]d-aspartate release above the effect of depolarising agent alone. This enhancement was blocked by the nicotinic antagonist mecamylamine. Other experiments revealed that in the absence of a depolarising stimulus, the nAChR agonists nicotine, epibatidine and anatoxin-a could evoke the release of [3H]d-aspartate in a Ca2+- and concentration-dependant manner. Differential sensitivity to the α7- and β2∗-selective nAChR antagonists α-bungarotoxin (α-Bgt) and dihydro-β-erythroidine (DHβE) implicated two nAChR subtypes (α7 and β2∗), and this was supported by using the subtype-selective agonists choline (10 mM; α7 selective, blocked by α-Bgt but not by DHβE) and 5-Iodo-A-85380 (10 nM; β2∗-selective, blocked by DHβE but not by α-Bgt). Immunocytochemistry showed that α-Bgt labelling was associated with structures immunopositive for vesicular glutamate transporters, in both frontal cortex sections and synaptosome preparations, supporting the presence of α7 nAChR on glutamatergic terminals in rat frontal cortex.